General Background
Faithful reproduction of color images on color printing devices relies on a color management system. Such a system transforms color values in an original document into colorant values for a printing system in such a way that the colors in the original document are faithfully rendered.
Most commercial color management systems are based on the specifications of the International Color Committee (ICC) for representing color transformations. The ICC has a website on http://www.color.org.
According to the ICC, a color transformation is stored in a “color profile”.
A “source color profile” contains a transformation for color or colorant values that serve as input—e.g. RGB values produced by a digital camera or from a previous color transformation—to color values in device independent color space, such as for example CIELAB.
A “destination color profile” contains a transformation from color values in a device independent color space to a set of color or colorant values—e.g. representing amounts of cyan (C), magenta (M), yellow (Y) and black (K) ink—that serve as an output, for example to a printer or to a next color or colorant transformation.
The determination of a color profile involves a characterization step whereby the device dependent color or colorant values are related to calorimetrically measured color values of color patches of a characterization target.
A key component of the ICC specification is the use of a well-defined “profile connection space” (PCS). This standard color space is the interface which provides an unambiguous connection between a source and a destination profile. It is the virtual destination for source profiles and the virtual source for destination profiles. If the source and destination profiles are based on the same PCS definition, even though they are created independently, they can be paired arbitrarily at run time by the color management system (CMM) and will yield consistent and predictable color reproduction from original to reproduction.
A source and a destination profile can be concatenated into a color link. Such a link provides a direct transformation from color data that serves as input to color data that serves as output. For example, a color link may directly transform RGB data from an image capturing device into CMYK data of a printing device.
Digital Proofing System
A digital proofing system is used to verify the contents and various technical characteristics including color reproduction of a digital print master by simulating the printed results.
This simulation involves transforming the colorant values of the print master to colors in a profile connection space by means of a source profile and then transforming these colors from the profile connection space to the colorant space of the proofing device by means of a destination profile.
The destination profile for a proofing device is easily obtained by means of a characterization step of the proofing device.
The Problem
Ideally, a source profile for a digital proofing method matches the destination profile that was used for calculating the colorant values in the print master.
In practical cases, however, usually no information on the latter destination profile is available.
It my not be known, for example, if said latter destination profile was selected to transform colors from the profile connection space to colorants for commercial offset, newspaper offset, flexographic or gravure digital print masters.
Prior Art Solutions
A prior art solution relies on the assumption that in practice many print masters are created using only a limited set of standardized color destination profiles.
By analyzing the colorant values in a set of separations of a digital print master, and comparing these values with the values produced by a number of known standardized destination profiles, it is possible to guess which destination profile has been used to calculate the colorant values in the print master.
A first prior art method of analyzing the colorant values in a set of separations of a print master involves looking for the maximum value of the sum of the colorant values. Standardized CMYK destination profiles, for example, produce separations having a maximum value of the sum of colorant values that is typically in the range from 240% to 340%, for example 300% in a particular case.
A second prior art method of analyzing the colorant values in a set of separations is to look for the maximum amount of black colorant. Standardized destination profiles produce a maximum amount of black colorant in the black separation that ranges from 70% to 100%, for example 90% in a particular case.
While the above methods to guess what destination profile has been used to create a digital print master work fine in many cases, they are all but full proof.
One can, for example imagine an image that does not contain any dark colors at all. In that case neither the maximum value of the sum of the colorants, nor the maximum amount of black colorant in the separations can provide a useful and reliable indicator of what destination profile was used.
A need exists for a method that enables to determine what destination profile was used to create a set of separations for a print master and that is reliable for a wide range of images.